Question

Heavy water ($\text{D}_2\text{O}$) acts as a moderator in nuclear reactors. **Why heavy water works:** Deuterium nuclei in heavy water have a mass comparable to that of a neutron, enabling efficient energy transfer through elastic collisions, thereby slowing neutrons effectively. **Why other options are incorrect:** - **(A) Lead shielding:** Lead is effective at blocking gamma radiation, not at slowing neutrons. - **(C) Heavy nuclei:** In an elastic collision, maximum energy transfer occurs when the colliding particles have equal mass. Heavy nuclei have much larger mass than neutrons, making energy transfer very inefficient. - **(D) Electric field:** Neutrons are electrically neutral and are unaffected by electric fields. **Answer: (B)**